1. Field of the Invention
The present invention relates to video reproduction, and more particularly, to an apparatus, method, and medium for video synchronization.
2. Description of the Related Art
Typical Moving Picture Experts Group (MPEG) sync scheme performs video synchronization according to information called a time stamp for decoding and reproducing input video pictures. After the synchronization of the video pictures, the MPEG sync scheme also performs picture Skipping/Repeating at uniform time intervals so as to prevent a decoding buffer from overflow/underflow. Picture Skipping denotes skipping a current picture to be transmitted and dealing with the next picture. Picture Repeating denotes reproducing a same picture repeatedly until a predetermined time. At this time, from video pictures input as IBBPBBP or IPPPP, only the B-type picture or the last picture of GOP (Group of Picture), excluding a reference picture, should be skipped.
FIG. 1 is a schematic block diagram of a conventional digital broadcast receiver.
Referring to FIG. 1, the digital broadcast receiver that receives and reproduces video signals includes a transport stream (TS) de-multiplexer 110, a digital phase locked loop (D-PLL) 120, comparators 130 and 140, a video decoder buffer 150, a video decoder 160, a frame reorder buffer 170, and a video display 180.
The TS de-multiplexer 110 receives transmitted streams, de-multiplexes a plurality of programs, and extracts parameters for video reproduction and synchronization. Here, the demultiplexed data and the extracted parameters may include encoded video streams to be forwarded to the video decoder buffer 150, a decoding time stamp DTS indicating when the video data should be decoded, and a presentation time stamp PTS indicating when the video data should be reproduced. A signal, denoted as PCR (Program Clock Reference), also output by the TS de-multiplexer 110, is time administration information for synchronizing a reference frequency of the receiver with that of the transmitter.
The D-PLL 120 receives the PCR and synchronizes the reference frequency of the receiver with that of the transmitter.
The comparator 130 compares the DTS indicating a decoding time of a video picture to a reference time, and generates a control signal for decoding time. A reference time may be referred to as a system time clock (STC).
The comparator 140 compares the PTS indicating a reproducing time of a video picture to a reference time (STC), and generates a control signal for reproduction time.
The video decoder buffer 150 receives and stores the de-multiplexed video stream from the TS de-multiplexer 110.
The video decoder 160 decodes video data received from the video decoder buffer 150 into a picture image. Here, the encoded video data is decoded at a designated time according to the DTS, and the resulting decoded video data is stored in a picture buffer (not shown).
The frame reorder buffer 170 re-orders decoded I-(type) pictures and P-(type) pictures received from the video decoder 160 and outputs the results to the video display 180. However, the frame reorder buffer 170 passes decoded B-pictures received from the video decoder 160 to the video display as they are.
The video display 180 displays I- and P-pictures reordered by the frame reorder buffer 170, and B-pictures output by the video decoder 160. That is, each picture data stored in the picture buffer is reproduced by the video display 180 at a designated time according to the PTS.
As such, conventional MPEG sync scheme performs video synchronization according to information called time stamps for video picture decoding and reproducing. By this sync scheme, video pictures are synchronized, and in order to prevent decoding buffer overflow/underflow, picture Skipping/Repeating is performed at uniform time intervals. At this time, from video pictures input as, for example, IBBPBBP, or IPPPP, only the B-picture or the final picture of GOP, excluding a reference picture used for forming other images, should be skipped, if skipping is needed for lip sync or the prevention of overflow/underflow. For this, it is necessary to predict pictures that will be skipped. However, a possible error in the prediction may lead to picture distortion.